What Causes Swelling,pain And Difficulty In Bending Finger?

About a 1 1/2 months ago I had swelling, pain and difficultly bending my pointer finger on my right hand. I went to a doctor, but he didn't do any x-rays and said it was probably tendonitis and gave me an injection of cortisone into the area and gave me a finger brace to wear at night. That was 3 weeks ago and it's not any better. Running my finger along the side of my knucke it feels very hard but swollen. When I rest it for any length of time, it becomes very stiff but after using it and bending it gently it seems to loosen up. Nonetheless, I have problems with it every day and the swelling hasn't completed subsided yet. I don't know if I've injured it, or could this just be arthritis?